Abstract

DNA uptake by competent H. influenzae cells requires the presence of a specific base sequence (uptake site) on the entering DNA duplex. This sequence is probably recognized by a receptor on the cell surface. We have examined the kinetics and stoichiometry of DNA uptake by competent cells and have shown that the results are consistent with a simple model involving: 1) reversible binding of the DNA uptake site to a cell surface receptor, 2) an irreversible step resulting in a commitment toward DNA uptake, and 3) transport of the DNA duplex into the cell. We have also shown that a competent H. influenzae cell can absorb only 4 to 8 molecules of DNA, regardless of their length. To explain this counting mechanism, we suggest that each cell has only 4 to 8 receptors and that each receptor can be used to transport only one molecule of DNA.